It is known that conjugated diene polymers comprise a mixture of recurring units of structures that in the art are referred to as (cis or trans) 1,4-structures and 1,2-structures. The 1,4-structure results in unsaturation in the polymer backbone, the 1,2-structure results in the attachment of unsaturated groups (vinyl groups) to the backbone. Conjugated diene polymers with selectively hydrogenated 1,2-butadiene (butylene) units have been found to have a number of desirable properties. For instance, the absence of vinyl groups results in improved heat resistance, while the remaining essentially unaffected unsaturation in the backbone results in good processability and good elastomeric properties at low temperature.
In U.S. Pat. No. 3,700,748 block polymers are provided having substantially improved capability of forming stable elastomeric polar derivatives, these block polymers being prepared by block polymerizing a monovinyl arene with butadiene, the butadiene block containing between about 8 and 80 percent 1,2-structure and thereafter selectively hydrogenating (in the presence of a nickel based catalyst) so as to substantially eliminate the unsaturation in the pendant vinyl groups. This process, however, is not very selective since it results in hydrogenating butadiene blocks having 1,2-structure, as well as those having a 1,4-structure.
U.S. Pat. No. 3,663,635, DE 3401983, U.S. Pat. No. 5,039,755, U.S. Pat. No. 5,132,372, EP 339986, EP 434469, EP 544304, EP 795564, EP 810231, and WO 9525130 described catalyst systems that are suitable for the hydrogenation of conjugated diene (co)polymers. These catalyst systems are prepared by reacting a titanocene or similar group 4 metallocene (i.e., a ferrocene-like molecule based on a group 4 metal and 2 η5 ligands), with a metal hydride or an organometallic compound and contacting (activating) the resulting catalyst mixture with hydrogen. These catalyst systems have a very high hydrogenation activity, but none are known to be capable of selectively hydrogenating said conjugated diene (co)polymers into partially hydrogenated (co)polymers.
In a paper, entitled “Metallocenes: homogeneous catalysts for elastomer hydrogenation”, the authors M. D. Parellada, J. A. Barrio, J. A. Delgado (Rev. R. Aced. Cienc. Exactas, Fis. Nat. Madrid (1993), 87(1), 127–9) describe the advantages of a metallocene type catalyst for hydrogenating styrenic block copolymers over Ziegler type hydrogenation catalysts. It is said that replacement of a cyclopentadienyl (Cp) ring by a pentamethylcyclopentadienyl (Cp*) ring in these titanium complexes brings about greater stability and more selectivity, first hydrogenating vinyl bonds rather than the olefin bonds within the copolymers. A suitable process for the preparation of partially hydrogenated polymers is, however, not disclosed.
EP 545844 describes a process for partial hydrogenation of a styrenic block copolymer in the presence of Cp*CpTiX2 and n-butyl lithium. Although all or nearly all of the 1,2-polybutadiene is hydrogenated, so is also a major portion of the 1,4-polybutadiene (cf. Examples 12 to 15). This application rather illustrates the difficulty of obtaining highly selective, partially hydrogenated polymers wherein the 1,2-butadiene recurring units are hydrogenated, but the 1,4-butadiene recurring units are not hydrogenated.
EP 584860 describes a process for partial hydrogenation of conjugated diene polymers, that selectively hydrogenates butadiene units, but not (or less so) isoprene units. This process too is not very selective as regards hydrogenating butadiene blocks having 1,2-structure, respectively 3,4-structure (in case of isoprene) only.
In JP 04096904 Asahi describes a method for hydrogenating olefin compounds, using a titanium-, zirconium- or hafnium-based metallocene compound as hydrogenation catalyst and in the presence of a reducing compound. For instance, this application describes the hydrogenation of 7 different (block) copolymers with Cp*2TiCl2 and dibutyl magnesium or n-butyl lithium as catalyst. Hydrogenation in each instance was nearly complete; preparation of the aforementioned partially hydrogenated conjugated diene polymers is not described.
In EP 302505 Asahi describes partially hydrogenated butadiene polymers or partially hydrogenated random butadiene/styrene copolymers and a process for preparing the same. However, although “any catalysts and any production methods may be utilized” (page 5, line 8), the specification also indicates that “the selective partial hydrogenation of the present invention must be practiced under extremely mild reaction conditions . . . ” (page 5, lines 12–13). It would therefore appear that the preparation of partially hydrogenated conjugated diene polymers requires rigid control of the reaction conditions. Besides, the hydrogenation illustrated in this patent is not selective to the 1,2-conjugated diene recurring unit: without even being close to complete hydrogenation of the “vinyl-linkage moiety [B]”, hydrogenation of the “whole butadiene portion [A]” is already greater than the “vinyl-linkage content [V]” (cf. Table 1 of this patent), whereas if the “vinyl-linkage moiety [B]” is completely hydrogenated, than also the 1,4-linkage content is substantially reduced (cf. Tables 5 to 8 of this patent).
In JP 09-291121, Asahi describes an improvement on said process, thus producing a partially hydrogenated butadiene polymer with selective hydrogenation of the 1,2-diene recurring units. According to this application, at least 90% of the 1,2-diene recurring units must be hydrogenated, with a catalyst having an r1/r2 ratio greater than 5, wherein ‘r1’ corresponds with the reaction rate for the hydrogenation of the 1,2-diene recurring units and wherein ‘r2’, at identical reaction conditions, corresponds with the reaction rate for the hydrogenation of the 1,4-diene recurring units. A compound comprising titanium ligated by a substituted metallocene is provided as an example of a suitable hydrogenation catalyst. However, stringent control of the hydrogenation reaction still appears to be necessary, to avoid the hydrogenation of the 1,4-diene recurring units.
In U.S. Pat. No. 5,925,717 the hydrogenation of hydrogen terminated SBS block copolymers is disclosed, in the presence of optionally substituted titanium indenyl and/or bis indenyl compounds as catalyst. Such catalysts are said to be equally active to 1,4-conjugated diene recurring units as 1,2-conjugated diene recurring units (column 6, lines 52–58).
It is therefore an object of the present invention to provide an attractive process, that does not require rigid control of the reaction conditions, to prepare the partially hydrogenated conjugated diene polymers described herein before.